Communication pubs.acs.org/JACS
Twisted Aspirin Crystals Xiaoyan Cui,† Andrew L. Rohl,‡ Alexander Shtukenberg,*,† and Bart Kahr*,† †
Department of Chemistry, New York University, 100 Washington Square East, Silver Center, Room 1001, New York, New York 10003, United States ‡ Department of Chemistry, Curtin University, P.O. Box U1987, Perth, Western Australia 6845, Australia ABSTRACT: Banded spherulites of aspirin have been crystallized from the melt in the presence of salicylic acid either generated from aspirin decomposition or added deliberately (2.6−35.9 mol %). Scanning electron microscopy, X-ray diffraction analysis, and optical polarimetry show that the spherulites are composed of helicoidal crystallites twisted along the ⟨010⟩ growth directions. Mueller matrix imaging reveals radial oscillations in not only linear birefringence, but also circular birefringence, whose origin is explained through slight (∼1.3°) but systematic splaying of individual lamellae in the film. Strain associated with the replacement of aspirin molecules by salicylic acid molecules in the crystal structure is computed to be large enough to work as the driving force for the twisting of crystallites.
Figure 1. Aspirin (salicylic acid concentration in the melt is 19 mol %) spherulites grown at 4 °C.
We have established previously that impurities are likely to be a necessary precondition for twisting.17−19 As such, we surmised that the hydrolytic decomposition product of aspirin that forms on melting, salicylic acid, was the agent that actualized the optical oscillations through twisting. In fact, the most effective way to grow banded aspirin spherulites is to mix 19 mol % salicylic acid (Mallinckrodt 99%, mp = 158 °C) with aspirin prior to melting. Films grown in this way were approximately 2 μm thick with a pitch of about 100 μm. Band spacing (pitch) decreases from ∼800 to 20 μm as the concentration of salicylic acid in the melt increases from 2.6 to 35.9 mol % (Figure 2). Nominally, pure aspirin spherulites are characterized by larger pitch of 1−2 mm. In this case, banding can arise from traces of salicylic acid formed during melting. The banded spherulites grew as either of the two known polymorphs of aspirin or as a mixture of the two; it is difficult to distinguish them by powder X-ray diffraction as they are very close in structure (monoclinic, P2 1 /c) and frequently intergrown.3,4,14 Crystal data, Form I20 (II)1: a = 11.242(7) Å (12.095(7) Å), b = 6.539(4) Å (6.491(4) Å), c = 11.245(9) Å (11.323(6) Å), β = 95.90(1)° (111.509(9)°). The previously measured optical properties of aspirin single crystals are most likely a good approximation or average of Forms I and/or II: nX = 1.5066, nY = 1.6464, and nZ = 1.6604 at 546 nm.21 nZ and the optical plane are normal to (010). nX and nY are nearly bisected by [001].21−25 The excess salicylic acid in the banded spherulites was sublimed by heating the films at 70 °C for 100 min without significant loss of aspirin.26 The samples were coated with an evaporated gold layer (3−6 nm) thick and imaged by scanning
M
ost chemists in training have synthesized and crystallized aspirin; ∼45 000 tons are manufactured and consumed annually. That said, it seems unlikely that the crystallization of aspirin will continue to surprise, but that is precisely what transpired with the discovery of a new polymorph in 2005,1 precipitated by the computational predictions of new crystalline forms.2 This work sparked a wave of new research into aspirin crystallization from solution.3−6 What of crystallization from the melt or the glassy state (Tg = 243 K)? Researchers have investigated amorphous aspirin of late but do not comment on how these preparations ultimately crystallize.7−14 Here, we aim to illustrate that aspirin crystals grow from the melt as helicoidal ribbons with pitches that vary from about 20 μm to larger than 1 mm depending on the growth conditions. The possibility of this unusual growth mode is suggested by the appearance of aspirin as the 90th entry in a table of 135 organic compounds in a rare book about Twisted Crystals.15 The most authoritative source on the crystallization of pharmaceuticals from the melt16 gives no hint whatsoever that aspirin might be of interest to those interested in nonclassical crystal growth mechanisms or pattern formation in nature. Crystalline aspirin (2−4 mg) powder (Sigma 99%, mp = 133.5 °C) was melted between a glass slide and a coverslip. Banded spherulites form at temperatures
